For the Pie:
In bowl, add flour and salt. Add butter and rub into flour mixture. Once combined, stir in cold water just until also combined (do not overwork). Form into 2 balls. Wrap each ball with plastic wrap. Let rest for at least 2 hours. Roll 1 ball out and place in pie tin. Roll other ball out and use for top crust.
For the Rhubarb:
Cook rhubarb and sugar (and strawberries if desired) in pot for 5 minutes. Turn off heat and add cold water and flour. When cool, add egg. Add to tart or pie crust and bake at 350 degrees for 20 minutes.